Surface acoustic wave filter and method

ABSTRACT

An electronic apparatus for receiving and filtering high frequency waves comprises a sequence of T electro-acoustic transducers each having multiple interdigital electrode fingers and a centerline located equidistant between its outermost electrode fingers. For a centerline-to-centerline distance of a first adjacent pair of transducers D 1 , and centerline-to-centerline distances of other adjacent pairs of transducers D 2  to D T-1 , then at least some of D 2  to D T-1  differ from D 1  by a non-integral number of wavelengths. The transducer array is symmetrical about a centerline and the number of fingers in each transducer varies, with an input or output transducer closer to either end of the sequence having more fingers than a like kind transducer nearer the center.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.07/793,925, filed Nov. 18, 1991, now U.S. Pat. No. 5,309,126, issued May3, 1994, which is assigned to the same assignee as the presentapplication.

FIELD OF THE INVENTION

The present invention concerns an improved means and method for surfaceacoustic wave filters and apparatus using such filters.

BACKGROUND OF THE INVENTION

Modern portable radios and other electronic equipment require filters ofvery compact form and good electrical properties. Surface acoustic wave(SAW) filters are especially desirable for such purposes.

SAW filters are known in the prior art. For example, an article byMeirion Lewis entitled, "Saw Filters Employing InterdigitatedInterdigital Transducers, IIDT", Ultrasonics Symposium, 1982, pages12-17, describes SAW filters using interdigital transducers ofinterdigital electrodes on piezo-electric materials, each transducerhaving the same number of electrode fingers. Further, U.S. Pat. No.4,492,940 to Hikita describes an acoustic surface wave bandpass filterusing a linear array of alternate I/O transducers arranged in sequence,each having multiple interdigital electrode fingers, where the number offingers in the input transducers and the number of fingers in the outputtransducers decreases from the center to the end of the array.Additionally, U.S. Pat. No. 4,649,357 to Nagai et al., describes asurface acoustic wave filter using a linear array of alternate I/Otransducers arranged in sequence, each having multiple interdigitalelectrode fingers, where the distances (along the array) between thecenterlines of adjacent transducers differ by an integral multiple (0,±1, ±2, etc.) of wavelengths, and U.S. Pat. No. 4,931,755 to Sakamoto etal., describes use of an auxiliary capacitive transducer or other shuntcapacitance to adjust the filter cut-off above the pass-band. Theabove-listed patents are incorporated herein by reference.

Despite the considerable effort by many researchers, numerous problemsremain in connection with SAW filters, as for example, setting theoptimum input/output (I/O) impedance levels, optimizing the spectralresponse, reducing internal reflections within the filter, andsimplifying the fabrication process. These features of SAW filtersaffect their utility and performance in electronic apparatus. Hence,there continues to be an ongoing need for improved filters and filterapplications.

As used herein the abbreviation SAW is intended to stand for "surfaceacoustic wave". As used herein the word "filter", singular or plural, isintended to include any element having a frequency dependent transferfunction, and the words "SAW filter", singular or plural, are intendedto refer to elements employing surface acoustic waves and having afrequency dependent transfer function.

SUMMARY OF THE INVENTION

The present invention provides an improved means and method forelectronic devices and apparatus including SAW devices, and especiallySAW filters and improved apparatus based thereon.

According to a first embodiment of the present invention, there isprovided an electronic apparatus comprising a SAW filter including, ingeneral terms, a substrate for propagating acoustic waves on which arearranged in a direction of wave propagation, a sequence of inputtransducers and output transducers, each transducer having a number ofinterdigitated electrode fingers, wherein beginning at one end of thesequence, the number of fingers in a first transducer is larger than thenumber of fingers in another transducer further along the sequence.

According to another embodiment of the present invention, there isprovided an electronic apparatus comprising a SAW filter having acenterline-to-centerline separation of an adjacent pair of transducersof the SAW filter which differs from a centerline-to-centerlineseparation of another adjacent pair of transducers by a non-integralnumber of wavelengths of the acoustic wave.

In a preferred embodiment of the SAW filter, beginning at one end of thesequence, the number of fingers in a first input (or output) transduceris larger than the number of fingers in another input (or output)transducer further along the sequence. The sequence is desirablysymmetric about a centerline normal to the direction of propagation ofthe acoustic wave. It is further desirable that where a transducer inthe sequence contains an even number of electrode fingers, an adjacenttransducer contains an odd number of electrode fingers.

There is further provided a method for forming an apparatus including aSAW filter, which method comprises providing a substrate for propagatingacoustic waves and providing on the substrate a sequence of inputtransducers and output transducers, wherein each transducer has a numberof interdigitated electrode fingers, wherein beginning at one end of thesequence, the number of fingers in a first transducer is larger than thenumber of fingers in another transducer located further along thesequence.

In a preferred embodiment of the method, the step of providing thetransducers comprises providing transducers having predeterminedcenterlines normal to the direction of acoustic wave propagation,wherein each centerline is located half way between outer edges of firstand last electrode fingers of the transducer, and wherein centerlines ofa first adjacent pair of transducers are a distance D₁ apart andcenterlines of other adjacent pairs of transducers are distances D₂ toD_(T-1) apart. Here, T is the number of transducers, and at least one ofD₂ to D_(T-1) differs from D₁ by a non-integral number of wavelengths.

It is further desirable that the step of providing the transducerscomprise providing an input transducer with an even number of electrodefingers and another input transducer with an odd number of electrodefingers, and providing an output transducer with an odd number ofelectrode fingers, or vice versa as regards the designations of inputand output.

In a still further embodiment, there is further provided an improvedradio having therein one or more of the above-described filters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly simplified schematic drawing of the transducerarrangement of a first SAW filter according to the prior art;

FIG. 2 is a highly simplified schematic drawing of the transducerarrangement of another SAW filter according to the prior art;

FIG. 3 is a highly simplified schematic drawing of the transducerarrangement of a SAW filter according to an embodiment of the presentinvention;

FIGS. 4 and 5 are plan views illustrating transducers such as are usedin the present invention, having odd and even numbers of electrodefingers, respectively, and showing the locations of the centerlinesthereof;

FIG. 6 is a plan view illustrating multiple transducers such as are usedin the present invention, arranged in a sequential array andillustrating the relationships between adjacent transducers, thecenterlines thereof and the direction of propagation of the acousticwave; and

FIG. 7 shows a simplified schematic diagram of a radio employingmultiple filters, as for example, one or more SAW filters according tothe present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly simplified schematic drawing of the transducerarrangement of SAW filter 20 according to the prior art as described,for example, by Nagai, supra. Filter 20 comprises input terminal 21connected to a plurality of input transducers 22 and output terminal 23connected to a plurality of output transducers 24. Each transducer 22,24 is composed of a number of interdigital metal fingers coupledrespectively to input 21 and output 23 and to ground 26. Inputtransducers 22 and output transducers 24 are themselves interdigital.Thus, filter array 20 is formed by a sequence of alternating input 22and output 24 transducers, each output transducer 24 having the samenumber of fingers and each being spaced from the other transducers 22,24 by distances 1_(i) =l₁ to l_(T-1), where T is the number oftransducers. Nagai teaches that it is desirable that the distances l_(i)be related such that for arbitrary distance l₁ =m₁ ·L where ml is apositive real number and L is the acoustic wavelength, then the otherdistances l₂ to l_(T-1) satisfy l.sub. i =l₁ +m_(i) ·L where m_(i) is anarbitrary integer (0, ±1, ±2, etc.), for i=2, 3, 4, . . . T-1. Thus, thedistances l_(i) must be equal or differ by an integral number ofwavelengths L.

FIG. 2 is a highly simplified schematic drawing of the transducerarrangement of another prior art SAW filter 30 according to Hikita,supra. Filter 30 has input terminal 31 connected to input transducers32, 33, 34 symmetrically arranged around the filter centerline 39.Filter output 35 is connected to output transducers 36, 37 which arealso grouped symmetrically around filter centerline 39. The transducersare also connected to ground lead 38. The interdigital array oftransducers makes filter 30.

Hikita teaches that the number of electrode fingers in the transducersdesirably taper as one moves from transducers located at or near thecenterline toward transducers located at or near the ends of filter 30.That is, if N₁, N₂, and N₃ are the number of fingers respectively ininput transducers 32, 33, 34 and M₁ and M₂ are the number of electrodefingers in output transducers 36, 37 respectively, then N₁ ≧N₂ ≧N₃ andM₁ ≧M₂.

While the above described prior art arrangements provide filteringaction, they still suffer from some disadvantages well known in the art.For example, power handling capability is limited, it is difficult tooptimize I/O impedance levels, and the spectral response of the filteris difficult to adjust. For this reason, such filters are stilldifficult to make and use or have less than the desired performance inelectronic circuits, especially radios.

The foregoing and other problems are ameliorated or overcome by thestructure and method of the present invention which is illustrated invarious forms in FIG. 3-7. Unless specifically noted otherwise, thedesignations "input" and "output" (and the abbreviation "I/O"), whethersingular or plural, with respect to the present invention are intendedto be arbitrary designations for purposes of illustration anddiscussion, and can be interchanged.

Refer now to FIG. 3, a highly simplified schematic drawing of thetransducer arrangement of SAW filter 40 according to an embodiment ofthe present invention. Filter 40 comprises input 41 coupled to aplurality of transducers 42, 42', 43, 43', 44, 44' having numbers ofinterdigital electrode fingers N_(i) denoted N₁, N₁ ', N₂, N₂ ', N₃, N₃', respectively. Filter 40 further comprises output 45 coupled totransducers 46, 47, 47', 48, 48', 49, 49' having numbers of interdigitalelectrode fingers M_(j) denoted M₁, M₂, M₂ ', M₃, M₃ ', M₄, M₄ ',respectively. Transducers 42-44' and 46-49' are arranged in a sequentialarray, so that the inputoutput transducers desirably alternate as oneprogresses along the array. For example, starting at one end, a sequenceof output transducers 49, 48, 47, 46, 47', 48', 49', alternate with asequence of input transducers 44, 43, 42, 42', 43', 44' to form thesequence output-input-output-input-output-input- etc., (e.g.,transducers 49, 44, 48, 43, 47, 42, etc.) making up the exemplary SAWdevice.

As indicated on FIG. 3, the transducer's electrode "fingers" areinterdigitated or interleaved and desirably perpendicular to direction52 of propagation of the acoustic wave. For a given transducer, theelectrode fingers facing in a first direction are coupled to the input(or output) and the remaining electrode fingers of that transducer(facing in the opposite direction) are coupled to a common terminal, asfor example ground connection 50.

The transducers themselves are also interleaved or interdigitated, thatis, the ground contacts to the transducers are on one side of the arrayfor the input transducers and on the other side of the array for theoutput transducers and the conductors leading to the input terminalapproach only from one side of the array and the conductors leading tothe output terminal approach from the other side of the array. This isnot essential but is desirable and provides a convenient conductorlayout.

Further, the transducers of filter 40 are desirably but not essentiallysymmetrical about centerline 51 passing through the center of filterarray 40 perpendicular to acoustic wave direction 52. For convenience ofexplanation and without intending to be limiting, symmetry aboutcenterline 51 is assumed in the discussion that follows and reference totransducers 42-49 is intended to include transducers havingcorresponding primed reference numbers.

The number of electrode fingers N_(i), M_(j) in the transducersdesirably varies according to the position of the transducer in thetransducer array. It has been found that excellent electrical propertiesare obtained when the numbers of electrodes in the transducers variessuch that the outermost transducers have more electrode fingers than theinner transducers. Starting at one end of the array and progressingtowards centerline 51, the number of fingers in the first input (oroutput) transducer is larger than the number of fingers in the nextencountered input (or output) transducer. In a preferred embodiment thenumber of fingers N_(i), M_(j) in the outermost input or outputtransducer is also larger than the number of fingers in the mostcentrally located input or output transducer. For example, N₃ ≧N₂ and/orM₄ ≧M₃, and preferably, N₃ ≧N₁ and/or M₄ ≧M₁.

The number of fingers in a central transducer may be an odd numberincluding an integer having a value in the range from between thirty andsixty. The odd number usefully is an integer having a value in the rangefrom between forty four and forty eight.

A first transducer immediately adjacent the central one desirably isseparated therefrom by a gap between an outermost edge of an outerelectrode of the central one and an outermost edge of an adjacent outerelectrode of the first transducer. The gap usefully includes a width inthe range from three fourths to one wavelength.

The first transducer immediately adjacent the central one is usefullyseparated therefrom by a gap between an outermost edge of an outerelectrode of the central one and an outermost edge of an adjacent outerelectrode of the first transducer. The gap usefully includes a width ofsubstantially eighty five percent of a wavelength.

Alternatively, the first transducer immediately adjacent the central oneis desirably separated therefrom by a gap between an outermost edge ofan outer electrode of the central one and an outermost edge of anadjacent outer electrode of the first transducer. The gap usefullyincludes a width of substantially ninety three percent of a wavelength.

The surface acoustic wave filter includes a sequence of T input andoutput transducers including a plurality of input transducers and aplurality of output transducers, each having multiple interdigitalelectrode fingers wherein T is the number of electro-acoustictransducers and T is desirably equal to an odd number between 6 and 25.

Alternatively, the surface acoustic wave filter includes a sequence of Tinput and output transducers including a plurality of input transducersand a plurality of output transducers, each having multiple interdigitalelectrode fingers wherein T is the number of electro-acoustictransducers and T is usefully equal to thirteen.

The following is an example of a filter having thirteen transducers(T=13) with a center frequency of about 911 MHz, a pass-band ofapproximately 35±2 MHz with stop-bands on either side, a pass-bandinsertion loss of about 3 dB and a stop-band rejection of at least 20dB. The exemplary filter is symmetrical about centerline 51. The numberof electrode fingers n in the various transducers is shown in Table Ibelow. As an aid to relating this example to the transducers in FIG. 3,the reference numerals of the transducers of FIG. 3 are shown inparenthesis.

                  TABLE I                                                         ______________________________________                                        NUMBER OF ELECTRODE FINGERS                                                   IN DIFFERENT TRANSDUCERS                                                      TRANSDUCER     n      TRANSDUCER     n                                        ______________________________________                                        N.sub.1 (42, 42')                                                                          =     61     M.sub.1 (46)                                                                             =   45                                   N.sub.2 (43, 43')                                                                          =     55     M.sub.2 (47, 47')                                                                        =   40                                   N.sub.3 (44, 44')                                                                          =     65     M.sub.3 (48, 48')                                                                        =   40                                                             M.sub.4 (49, 49')                                                                        =   48                                   ______________________________________                                    

The numbers of fingers in the primed and unprimed transducers are equalin this example. This is preferred.

It is also desirable that the distance between the centerlines ofadjacent pairs of transducers be adjusted so as to not differ by anintegral multiple of acoustic wavelengths. This is explained below inmore detail.

As shown on FIG. 3, adjacent pairs of transducers, e.g., 49/44, 44/48,48/43, 43/47, 47/42 and 42/46, are separated by transducercenterline-to-centerline distances d_(i), which are in general notequal. Where T is the total number of transducers in array 40, thedistances d_(i) =d₁, d₂, d₃, . . . d_(T-1) for i=1, 2, 3, . . . T-1 arenot necessarily equal. With respect to the parameter d, the index "i" isintended to be arbitrary in the sense that any of the distances d_(i)can be assigned as i=1, and any other as i=2, and so on without aparticular order or sequence being implied. Determination of thelocations of the centerlines of the transducers is explained more fullyin connection with FIGS. 4-6.

In another embodiment, a surface acoustic wave filter usefully includesa sequence of T electro-acoustic transducers including a plurality ofinput transducers and a plurality of output transducers. Each transducerincludes multiple interdigital electrode fingers wherein T is the numberof electro-acoustic transducers. Each electro-acoustic transducer has apredetermined number of interdigital electrode fingers and eachelectro-acoustic transducer is either an input or an output transducer.An input transducer closer to an end of the sequence has moreinterdigital electrode fingers than another input transducer closer to acenter of the sequence, or an output transducer closer to an end of thesequence has more interdigital electrode fingers than another outputtransducer closer to the center of the sequence. A central transducerhas an odd number of interdigital electrode fingers wherein the oddnumber is an odd number between thirty four and sixty.

The odd number desirably but not essentially is an odd number chosenfrom the group consisting of forty-five and forty-seven.

In one embodiment, the transducer adjacent the central transducerdesirably includes a number of electrodes, the number chosen from thegroup consisting of sixty-one and sixty-five.

FIG. 4 is a plan view illustrating transducer 60 having an odd number ofelectrode fingers therein. Electrode fingers 62 extend inwardly in afirst direction from first common connection 64 and electrode fingers 63extend inwardly in the opposite direction from common connection 65.Fingers 62, 63 are interdigital, that is, they interleave. Electrodefingers 62, 63 are typically one-quarter wavelength wide and aretypically spaced apart by about one-quarter wavelength so that anelectrical potential can be created therebetween. Transducer 60 haswidth 66 extending between outer-most edges 621, 621' of fingers 622,622' of finger group 62 extending from connection 64. Transducer 60 hascenterline 67 which is normal to wave propagation direction 52.Centerline 67 is half way between the outermost edges of fingers 621,622, that is, the distances 68, 68' are equal. It will be noted that fora transducer with an odd number of fingers, centerline 67 falls on anelectrode finger.

The number n of electrode fingers in transducer 60 is defined as thenumber of fingers extending from common connection 64 plus the number ofelectrode fingers extending from common connection 65. While transducer60 is shown as having n=17, this is merely for convenience ofillustration and not intended to be limiting and larger or smallernumbers of fingers may be used, as is shown for example in Table Iabove. Transducer 60 is intended to illustrate a convenient transducerhaving multiple electrode fingers where n is an odd number.

In an alternative embodiment, a method for forming an electronicapparatus comprising a filtering function, includes steps of (i)providing a substrate for propagating acoustic waves and (ii) disposingon the substrate a sequence of input transducers and output transducers.Each transducer includes a number of interdigitated electrode fingers,wherein the number of fingers in a first input or output transducer ator near a first end of the sequence is larger than the number of fingersin another input or output transducer more remote from the first end.

The disposing step desirably includes a step of disposing a centraltransducer in the sequence of input and output transducers. The centraltransducer usefully comprises an odd number of electrodes.

The step of disposing a central transducer in the sequence of input andoutput transducers usefully includes a step of disposing a centraltransducer including a number of electrodes. The number is desirablychosen from the group of forty-five and forty-seven.

FIG. 5 is a plan view illustrating transducer 70 having an even numbersof electrode fingers therein. Electrode fingers 72 extend inwardly in afirst direction from first common connection 74 and electrode fingers 73extend inwardly in the opposite direction from common connection 75.Fingers 72, 73 are interdigital, that is, they interleave. Electrodefingers 72, 73 are typically one-quarter wavelength wide and aretypically spaced apart by about one-quarter wavelength so that anelectrical potential can be created therebetween. Transducer 70 haswidth 76 extending between outer-most edges 721, 731 of fingers 722,732. Finger 722 belongs to finger group 72 extending from commonconnection 74 and finger 732 belongs to finger group 73 extending fromcommon connection 75.

Transducer 70 has centerline 77 which is normal to wave propagationdirection 52. Centerline 77 is half way between the outermost edges 721,731 of fingers 721, 731, that is, the distances 78, 78' are equal. Itwill be noted that for a transducer with an even number of fingers,centerline 77 falls between electrode fingers in space 79.

The number n of electrode fingers in transducer 70 is defined as thenumber of fingers extending from common connection 74 plus the number ofelectrode fingers extending from common connection 75. While transducer70 is shown as having n=16, this is merely for convenience ofillustration and not intended to be limiting and larger or smallernumbers of fingers may be used, as is illustrated for example in Table Iabove. Transducer 70 is intended to illustrate a convenient transducerhaving multiple electrode fingers where n is an even number.

FIG. 6 is a plan view illustrating multiple transducers such as are usedin the present invention, arranged in a sequential array 80 andillustrating the relationships between adjacent transducers 60, 70, 60',the centerlines thereof 67, 77, 67' and direction 52 of propagation ofthe acoustic wave. FIG. 6 shows transducer array 80 making up a portionof filter 40 (FIG. 3). Array 80 comprises even number electrodetransducer 70 sandwiched between odd-number electrode transducers 60,60', such as are shown for example in FIGS. 3-5. Transducers 60 (odd),70 (even), 60' (odd) in FIG. 6 are intended to exemplify various of thetransducers in the filter of FIG. 3 having various numbers n ofelectrode fingers which are not necessarily the same. For example,transducers 60, 70, 60' of FIG. 6 exemplify transducers 42 (e.g., n=61),47 (e.g., n=40) and 43 (e.g., n=55) respectively and in either order, ortransducers 43 (e.g., n=55), 48 (e.g., n=40) and 44 (e.g., n=65)respectively and in either order, or their primed counterparts.Corresponding to the arrangement of FIG. 3, connections 64', 75, 64indicated by the letter "G" are coupled to common or ground lead 50 offilter 40, and connection 74 to output 51 and connections 65', 65 toinput 41 of filter 40.

Transducers 60, 60' have widths 66, 66' and centerlines 67, 67' definedin the same manner as for FIG. 4, that is, distances 680 and 681 areequal and distances 680' and 681' are equal. Transducer 70 has width 76and centerline 77 defined in the same manner as for FIG. 5, that is,distances 780 and 781 are equal. Centerlines 67, 77, 67' are separatedby distances 81 and 82 respectively. Analogous distances to othertransducers 700, 700' lying to the left and right, respectively, oftransducers 60, 60' are denoted as 82' and 81' respectively. In FIG. 6transducers 700 and 700' are illustrated as being further transducerswith n even. However, this is merely for convenience of illustration andnot intended to be limiting, since adjoining further transducers mayhave n even or odd.

Distances 81', 82, 81, 82' illustrate the manner in which distancesd_(i) in FIG. 3 are intended to be measured, that is, between thecenterlines of the respective adjacent transducers. In the exemplary 13transducer filter described earlier in connection with Table I and FIG.3, the preferred centerline-to-centerline (CTC) distances are typicallyas shown in Table II:

                  TABLE II                                                        ______________________________________                                        TRANSDUCER PAIR                                                               REFERENCE NUMBERS   CTC DISTANCES d.sub.i                                     ______________________________________                                        46-42               m.sub.1 · L + x                                  42-47               m.sub.2 · L + x - L/4                            47-43               m.sub.3 · L + x + L/4                            43-48               m.sub.4 · L + x - L/4                            48-44               m.sub.5 · L + x + L/4                            44-49               m.sub.6 · L + x - L/4.                           ______________________________________                                    

The CTC distances for the primed reference numbers are the same. Thequantity x is a design parameter and a positive real number. Thequantity L is the acoustic wavelength and m₁. . . m₆ are integers.Values of x about x=0.4L are suitable, with values in the range of about0.2L to 0.6L being useful.

Useful values of m₁ . . . m₆ are determined by the number of fingers inadjacent transducers and their separation. There must be sufficientspace between centerlines to at least accommodate the numbers of fingersin the adjacent transducers. Typically, the finger width and fingerseparation in the direction of wave propagation are about L/4 so thatthe distance from the leading edge of one finger to the leading edge ofthe next finger is about L/2. The larger the number n of fingers, thelarger must be the CTC spacing of adjacent transducers. Hence, minimumvalues of m₁ . . . m₆ are related to d_(i) and n. Those of skill in theart will understand how to determine such minimum values. In general, itis desirable that m₁ . . . m₆ be close to or equal to the minimumvalues.

As shown in Table II, at least some of the CTC distances d_(i) of thepreferred embodiment of the present invention differ by non-integralmultiples of the acoustic wavelength L, irrespective of which distanced_(i) is chosen as a basis for comparison.

Table III provides a set of values for number of finger pairs N_(xy) andinter-transducer spacings S_(xy) (measured between outer edges ofoutermost fingers, i.e., analogous to outer edges 621, 621' and 721, 731of FIGS. 4 and 5, respectively) for a particular filter constructed on64° LiNbO₃ (i.e., having Euler angles of 0, 26, 0), having a 1.35micrometer (5.8 micrometer wavelength) inter-electrode spacing withinthe transducers and employing 0.15 micrometer thick aluminummetallization to provide a 4.5 dB bandwidth (relative to zero insertionloss, not center frequency insertion loss) of 30 MHz about a centerfrequency of 836.5 MHz and a center frequency insertion loss of 2.1 dB.

                  TABLE III                                                       ______________________________________                                        S.sub.xy SPACING     x       N.sub.x                                                                             y     N.sub.y                              ______________________________________                                        S.sub.49,44                                                                            5.03192     49      50    44    69                                   S.sub.44,48                                                                            7.73197     44      69    48    42                                   S.sub.48,43                                                                            5.03192     48      42    43    59                                   S.sub.43,47                                                                            7.73197     43      59    47    42                                   S.sub.47,42                                                                            5.03192     47      42    42    65                                   S.sub.42,46                                                                            5.03192     42      65    46    47                                   ______________________________________                                    

The subscripts x and y refer to the transducer numbers assigned in FIG.3 and associated text, for a 13 transducer structure that is essentiallysymmetrical about centerline 51. Input and output impedances are 50 Ωand the finger overlap (i.e., transverse to centerline 52) is roughly 79micrometers or about 13.6 wavelengths.

Table IV provides another set of values (see also Table I) for number offinger pairs N_(xy) and inter-transducer spacings S_(xy) (measuredbetween outer edges of outermost fingers, i.e., analogous to outer edges621, 621' and 721, 731 of FIGS. 4 and 5, respectively) for a particularfilter also constructed on 64° LiNbO₃, having a 1.234 micrometer (4.936micrometer wavelength) inter-electrode spacing within the transducersand employing 0.15 micrometer thick aluminum metallization to provide a4.5 dB bandwidth (relative to zero insertion loss, not center frequencyinsertion loss) of 30 MHz about a center frequency of 911 MHz and acenter frequency insertion loss of 2.1 dB.

                  TABLE IV                                                        ______________________________________                                        S.sub.xy SPACING     x       N.sub.x                                                                             y     N.sub.y                              ______________________________________                                        S.sub.49,44                                                                            4.6         49      48    44    65                                   S.sub.44,48                                                                            7.07        44      65    48    40                                   S.sub.48,43                                                                            4.6         48      40    43    55                                   S.sub.43,47                                                                            7.07        43      55    47    40                                   S.sub.47,42                                                                            4.6         47      40    42    61                                   S.sub.42,46                                                                            4.6         42      61    46    45                                   ______________________________________                                    

The subscripts x and y refer to the transducer numbers assigned in FIG.3 and associated text, for a 13 transducer structure that is essentiallysymmetrical about centerline 51. Input and output impedances are 50 Ωand the finger overlap (i.e., transverse to centerline 52) is roughly72.6 micrometers or about 14.7 wavelengths.

Similar filters may also be built on other high-coupling cuts (e.g.,coupling coefficient greater than 5%) of LiNbO₃, LiTaO₃ and othermaterials, e.g., 36° LiTaO₃, 41° LiNbO₃, 64° LiNbO₃ and the like.

In FIG. 6, the "ground" of "common" terminals of the transducers lie onopposite sides of the longitudinal axis (parallel to propagationdirection 52) of filter 80. These ground terminals are desirably allconnected together by "hooking" or connecting the last ground finger ofone transducer to the oppositely lying ground electrode of the nexttransducer. For example, left-most finger 622 of transducer 60, whichextends from ground potential electrode 64, is connected by longitudinalconductor 751 to "ground" potential electrode 75 of transducer 70.Similarly, right-most electrode 622' which extends from "ground"potential electrode 64' of transducer 60' is connected to "ground"potential electrode 75 of transducer 70 by longitudinal conductor 752.

FIG. 7 is a simplified schematic diagram indicating how theabove-described filters according to the present invention areadvantageously used in radio circuit 90. Radio circuit 90 comprisesantenna 91 sending or receiving signals from optional diplexer 92 (whichis coupled to the transmitter if one is used). Diplexer 92 sends anincoming signal to filter 93 which passes the resulting band limitedsignal to amplifier 94. From amplifier 94 the signal passes throughanother band limiting filter 95 to mixer 96. Mixer 96 also receives asignal from local oscillator 97 through band limiting filter 98. Themixed signal from mixer 96 passes through band limiting filter 99 whichremoves the local oscillator signal and sends the result to the receiverIF via output 100. Filter 101 cleans up the signal from the transmitterto reduce any surious out-of-band signals before they are amplified bypower amplifier 102 and then directed via diplexer 92 to antenna 91 fortransmission. Filter 101 is preferrably a filter of the type describedhere and made according to the structure and method of the presentinvention.

Filters 93, 95, 98 and/or 101 are advantageously SAW filters of the typedescribed here and made according to the structure and method of thepresent invention, but of varying frequency or other propertiesaccording to the particular desired function. For example, filter 93removes input RF frequencies outside the band in which the receiver isintended to operate. This is particularly useful for narrow bandreceivers such as are frequently required in cellular phone and pagingapplications and the like.

Filter 95 may have the same or a different pass band than filter 93 andremoves any undesired harmonics generated by amplifier 94 or otherout-of-band signals not removed by filter 93. Filter 98 desirably passesthe LO frequency and stops undesired harmonics thereof. Filter 99desirably passes the difference frequencies produced by mixer 96, andblocks the local oscillator and input RF frequencies. This is importantto avoid saturating the input stages of the IF amplifier which istypically coupled to output 100. Thus, electronic apparatus, andespecially radios have improved properties as a result of the improvedproperties of the SAW filters of the present invention.

The above-described filters are constructed by a method comprising, in afirst embodiment, providing a substrate for propagating acoustic wavesand providing on the substrate a sequence of input transducers andoutput transducers, each transducer having a number of interdigitatedelectrode fingers, wherein the number of fingers in a first input oroutput transducer at or near a first end of the sequence is larger thanthe number of fingers in another input or output transducer more remotefrom the first end. Lithium niobate and lithium tantalate are examplesof suitable substrate materials, with the former being preferred. Thetransducers are formed by depositing a conductive film, usually a metalsuch as aluminum, on the substrate and then by means of photo-maskingand etching steps well known in the art, the unwanted portions of themetal film are removed. These metallization, masking and etching stepsare conventional and well known in the art. What is different is theshape of the electrode pattern formed in the film. This is accomplishedby providing an etch using an etch mask having an image of the electrodeshape and arrangement described herein.

In a further embodiment there is provided a method comprising, providinga substrate for propagating acoustic waves and providing on thesubstrate a sequence of transducers having predetermined centerlinesnormal to the direction of acoustic wave propagation, wherein eachcenterline is located half way between outer edges of first and lastelectrode fingers of each transducer, and wherein centerlines of a firstadjacent pair of transducers are a distance D₁ apart and centerlines ofother adjacent pairs of transducers are distances D₂ to D_(T-1) apart,where T is the number of transducers, and wherein at least one of D₂ toD_(T-1) differs from D₁ by a non-integral number of wavelengths. It isfurther desirable that the foregoing step of providing transducershaving the above-described centerline spacing and the step of providingtransducers having numbers of fingers that vary along the transducersequence in the above-described manner, be carried out simultaneously asa part of the metallization, masking and etching steps to define thetransducer conductor pattern on the substrate.

It is further desirable that the step of providing the transducercomprises, providing a number of fingers in a first input (or output)transducer located at an end of the transducer array which is largerthan a number of fingers in another input (or output) transducer furtheralong the sequence, and in further detail, providing a sequence oftransducers symmetrical about a centerline and wherein the number offingers in a first input (or output) transducer located near an end ofthe array is larger than the number of fingers in another input (oroutput) transducer located nearest the centerline.

In a preferred embodiment, the step of providing the transducerscomprises providing one transducer in the sequence containing an oddnumber of electrode fingers and an adjacent transducer containing aneven number of electrode fingers, and further that the step of providingthe transducers comprises providing an input transducer with an evennumbers of electrode fingers and an output transducer with an odd numberof electrode fingers, or vice versa as regards the designations of inputand output. In a still further embodiment, the step of providing thetransducers comprises providing an input transducer with an even numberof electrode fingers and another input transducer with an odd number ofelectrode fingers, and providing an output transducer with an odd numberof electrode fingers, or vice versa as regards the designations of inputand output.

Based on the foregoing description, it will be apparent to those ofskill in the art that the present invention solves the problems andachieves the goals set forth earlier, and has substantial advantages aspointed out herein, namely, having larger numbers of electrode fingerson the outboard transducers compared to the inboard transducers improvesthe power handling capability. Further, the arrangement of the presentdesign provides greater design flexibility in optimizing the impedanceand spectral response.

While the present invention has been described in terms of particularmaterials, structures and steps, these choices are for convenience ofexplanation and not intended to be limiting and, as those of skill inthe art will understand based on the description herein, the presentinvention applies to other choices of materials, arrangements andprocess steps, and it is intended to include in the claims that follow,these and other variations as will occur to those of skill in the artbased on the present disclosure.

I claim:
 1. An electronic apparatus comprising a surface acoustic wave filter including a substrate comprising sixty four degree lithium niobate for propagating surface acoustic waves on which transducers are arrayed in a direction in which surface acoustic waves travel to form an array, the transducers comprising a plurality of input and a plurality of output transducers, each input and output transducer having a number of electrodes, wherein at least one of the input or output transducers located near an end of the array has a greater number of electrodes as compared to another input transducer located more centrally in the array, wherein a centerline-to-centerline separation of an adjacent pair of transducers differs from a centerline-to-centerline separation of another adjacent pair of transducers by a non-integral number of wavelengths of the surface acoustic wave, wherein a centerline of a transducer is located half way between outer edges of outer-most electrodes of that transducer.
 2. An electronic apparatus as claimed in claim 1, wherein a central one of said input and output transducers includes an odd number of electrodes.
 3. An electronic apparatus as claimed in claim 2, wherein said odd number comprises an integer having a value in the range from between thirty and sixty.
 4. An electronic apparatus as claimed in claim 2, wherein said odd number comprises an integer having a value in the range from between forty four and forty eight.
 5. An electronic apparatus as claimed in claim 2, wherein a first transducer immediately adjacent said central one is separated therefrom by a gap between an outermost edge of an outer electrode of said central one and an outermost edge of an adjacent outer electrode of said first transducer, said gap having a width in the range from three fourths to one wavelength.
 6. An electronic apparatus as claimed in claim 2, wherein a first transducer immediately adjacent said central one is separated therefrom by a gap between an outermost edge of an outer electrode of said central one and an outermost edge of an adjacent outer electrode of said first transducer, said gap having a width of substantially eighty five percent of a wavelength.
 7. An electronic apparatus as claimed in claim 2, wherein a first transducer immediately adjacent said central one is separated therefrom by a gap between an outermost edge of an outer electrode of said central one and an outermost edge of an adjacent outer electrode of said first transducer, said gap having a width of substantially ninety three percent of a wavelength.
 8. An electronic apparatus as claimed in claim 2, wherein said surface acoustic wave filter includes a sequence of T input and output transducers including a plurality of input transducers and a plurality of output transducers, each having multiple interdigital electrode fingers wherein T is the number of electro-acoustic transducers and T is equal to an odd number between 6 and
 25. 9. An electronic apparatus as claimed in claim 2, wherein said surface acoustic wave filter includes a sequence of T input and output transducers including a plurality of input transducers and a plurality of output transducers, each having multiple interdigital electrode fingers wherein T is the number of electro-acoustic transducers and T is equal to thirteen.
 10. A surface acoustic wave filter comprising a sequence of T electro-acoustic transducers including a plurality of input transducers and a plurality of output transducers, each having multiple interdigital electrode fingers wherein T is the number of electro-acoustic transducers, and each electro-acoustic transducer has a predetermined number of interdigital electrode fingers, each electro-acoustic transducer comprising either an input or an output transducer, and an output transducer closer to an end of the sequence has more interdigital electrode fingers than another output transducer closer to the center of the sequence, a central transducer has an odd number of interdigital electrode fingers wherein said odd number is an odd number between thirty four and sixty and wherein a centerline-to-centerline separation of a first adjacent pair of transducers differs from a centerline-to-centerline separation of a second adjacent pair of transducers by a non-integral number of wavelengths of the surface acoustic wave, wherein a centerline of a transducer is located half way between outer edges of outer-most electrodes of that transducer.
 11. A surface acoustic wave filter as claimed in claim 10, wherein T is an odd number.
 12. A surface acoustic wave filter as claimed in claim 10, wherein said sequence of T electro-acoustic transducers are disposed on a substrate, said substrate including a substrate chosen from the group consisting of forty-one degree lithium niobate, thirty-six degree lithium tantalate and sixty-four degree lithium niobate.
 13. A surface acoustic wave filter as claimed in claim 10, wherein said odd number is an odd number chosen from the group consisting of forty-five and forty-seven.
 14. A surface acoustic wave filter as claimed in claim 10, wherein a transducer adjacent said central transducer includes a number of electrodes, said number chosen from the group consisting of sixty-one and sixty-five.
 15. A method for forming an electronic apparatus comprising a filtering function, comprising steps of:providing a substrate for propagating acoustic waves; and disposing on the substrate a sequence of input transducers and output transducers, each transducer having a number of interdigitated electrode fingers, wherein the number of fingers in a first input transducer at or near a first end of the sequence is larger than the number of fingers in another input transducer more remote from the first end, wherein a centerline-to-centerline separation of a first adjacent pair of transducers differs from a centerline-to-centerline separation of a second adjacent pair of transducers by a non-integral number of wavelengths of the surface acoustic wave, wherein a centerline of a transducer is located half way between outer edges of outer-most electrodes of that transducer.
 16. A method as claimed in claim 15, wherein said providing step includes a step of providing a substrate chosen from the group consisting of forty-one degree lithium niobate, thirty-six degree lithium tantalate and sixty-four degree lithium niobate.
 17. A method as claimed in claim 15, wherein said disposing step includes a step of disposing a central transducer in said sequence of input and output transducers, wherein said central transducer comprises an odd number of electrodes.
 18. A method as claimed in claim 15, wherein said step of disposing a central transducer in said sequence of input and output transducers includes a step of disposing a central transducer including a number of electrodes wherein said number is chosen from the group of forty-five and/forty-seven.
 19. A method as claimed in claim 15, wherein said disposing step includes steps of:disposing a central transducer in said sequence of input and output transducers, wherein said central transducer is an input transducer; and disposing an output transducer adjacent said central transducer, wherein said adjacent output transducer includes a number of electrodes and wherein said number is chosen from the group consisting of sixty-one, sixty-three and sixty-five.
 20. A method as claimed in claim 15, wherein said disposing step includes a step of disposing thirteen transducers in a linear array, wherein, starting from either end of said linear array, a first transducer includes forty-eight electrodes, a second transducer includes sixty-five electrodes, a third transducer includes forty electrodes, a fourth transducer includes fifty-five electrodes, a fifth transducer includes forty electrodes, a sixth transducer includes sixty-one electrodes and a seventh transducer includes forty-five electrodes. 